Stabilized non-linear feedback amplifier



May 17, 1966 v. R. SAARI STABILIZED NONLINEAR FEEDBACK AMPLIFIER FiledOct. 5, 1963 ATTORNEY lNI/ENTOP KR. SAAR/ BV iM United States Patent"ice 3,252,007 STABILEZED NON-LINEAR FEEDBACK AMPLIFIER Veikko R. Saari,Murray Hiil, N.J., assignor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a

corporation of New York 1 Filed Oct. 3, 1963, Ser. No. 313,650 5 Claims.(Cl. -30788.5)

This invention relates to feedback amplifiers and more particularly tofeedback amplifiers employing non-linear negative feedback.

Amplifiers providing complex or non-linear input-out put characteristicsare often required as limiters, volume compressors and the like. Byemploying negative feedback that varies in magnitude with the signallevel it is possible to achieve quite a variety of input-outputcharacteristics, including logarithmic. One limitation in the design ofsuch amplifiers is that the ,u.,8 characteristics, which is, of course,dependent on the transfer characteristic of the forward pa-th (,u) andthat of the feedback path ([3), must be so designed that the amplifieris stable for all values of ,8 which, of course, varies with signallevel. Thus, if conditions are adjusted for a maximum bandwidth for thecondition of a minimum value of B, any increase in ,8 which may berequired at other signal levels may and usually will seriously impairthe stability of the amplifier. Accordingly, it has been necessary thatthe bandwidth and related characteristics be determined by therequirements for stable operation under the signal conditions for which[3 is a maximum.

It is an object of the invention to reduce the limitations on bandwidthand other operating characteristics of amplifiers employing non-linearnegative feedback while maintaining the stability of operation of theamplifier.

In accordance with this invention, an amplifier having a non-linearinput-output characeristic which is achieved by employing non-linearnegative feedback is arranged to have the nominal gain vary in a fashioncomplementary to the variation in the feedback fraction 8) required toproduce the non-linear input-output characteristic. Thus the feedbackfactor 13)v is maintained essentially constant and the requirements forstability of operation are the same for all signal levels. At the sametime, by employing a large feedback factor (,uB) the resultant overallgain of the amplifier, which is proportional to the reciprocal of B, canbe controlled to provide the required non-linear relationship betweensignal input and output by causing the feed-back fraction (5) to varynonlinearly with the signal level.

In a particular embodiment of the invention there is employed amultistage amplifier with at least one overall negative feedback networkincluding a biased switching diode which operates to bring the feedbacknetwork in and out of operation as the signal level varies. At the sametime, the overall nominal gain (a) is caused to vary to maintain thefeedback factor (MB) essentially constant by providing variable internalnegative feedback for at tion; and

FIG. 2 is a schematic diagram of a non-linear feedback amplifierembodying the invention which has a logarithmic input-output gaincharacteristic.

In the circuit of FIG. 1 an input signal from a source 3,252,007Patented May 17, 1966 10 is applied to a feedback amplifier. Thefeedback amplifier comprises an amplifier section 11 having a nominalgain ,u, and a ,8 feedback network 12. For illustrative purposes theamplifier 11 is shown as comprising three stages of amplifications 13,14 and 15 with one of the stages 15 having associated with it two local[3 feedback networks 16 and 17. A switch 19 is provided to connect oneor the other of the 6 feedback networks 16, 17 between the outputterminal 20 of amplifier 15 and its input. For low level output signalsbelow a predetermined value local feedback circuit 16 is connectedbetween the input and output terminals of amplifier 15. As is well knownin the art, the nominal gain (i.e., the 'gain without the feedbacknetwork 12) of such a three stage amplifier is where 1. 11. and 11 arethe nominal gains of amplifiers 13, 14 and 15, respectively, and 5 isthe fraction of the output signal from amplifier 15 which is fed 'backto the input of amplifier 15 through feedback circuit 16.

The output terminal 20 of amplifier 11 is also connected to a switch 25which connects either feedback network 26 or 27 between the outputterminal 20 of amplifier 11 and its input terminal to provide overallfeedback. At low levels of the output signal at output terminal 20,feedback network 26 is connected bymeans of switch 25 between the inputand output terminals of amplifier 11 to provide overall feedback. Inorder to obtain an inputoutput characteristic in which the outputincreases with input at a lower rate for signals above a predeterminedlevel, the value of the overall feedback fraction [3 for such higherlevel signals may be increased at the predetermined signal level bysubstituting feedback network 27 for the network 26 between the inputand output terminals of amplifier 11. If the amplifier has been designedclose to the requirements for stability at the lower value .of [3utilized for lower signal levels, the increase in [3 by the operation ofswitch 25 may cause such an increase in 8 that the amplifier may well.be brought into a condition of instability and oscillation may set in.In order to avoid this instability designers, heretofore, have had toconsider all possible values of B and accept the narrower availablebandwidth afforded with the higher value Of 80.

In accordance with applicants invention, however, instability is avoidedby reducing the u, or nominal gain, of the amplifier when the overallfeedback fraction 5 is increased so that the feedback factor 8 issubstantially constant irrespective of which overall feedback network 26or 27 is connected. To accomplish this, switch 19,

where 5 is the feedback fraction of the local feedback path between theinput and output terminals of amplifier 15 and n3 is the nominal gain ofamplifier 15. The overall nominal gain of the amplifier is u: i 1 H3Biand the feedback factor for the overall loop is:

where 5 is the feedback fraction of the overall feedback path so thatwhere 6 l .maintained constant and the stability of the amplifier ismaintained. In this way the bandwidth of the amplifier may be maintainedat the value at which stable operation i is attained for the lower valueof the feedback fraction {i even though the feedback is increased athigher signal levels to give the non-linear type of amplificationdesired.

It should be recognized, of course, that when it is desired that thegain of the feedback amplifier be increased for signals above apredetermined level, the overall feedback fraction 13 may be decreasedand if it is desirable to maintain the feedback factor constant this canbe achieved by increasing the nominal gain t.

Finally, switches 19 and 25 have been shown in me chanical form in orderto simplify the explanation of the circuitry. As is well known in theart, many forms of high speed electronic switches may be employed forthis purpose.

An amplifier having a logarithmic input-output characteristic andembodying the principles of this invention is shown in FIG. 2. In orderto obtain such an inputoutput characteristic it is necessary to employincreasing gain of the amplifier so that the feedback factor p.5 is heldrelatively constant and stability is maintained. In

this particular embodiment of the invention ,ufi is not, as will beseen, held absolutely. constant; rather ,u is reduced as ,6 is increasedto hold the product 8 within tolerable limits.

of ,u as fi 'increases so that and 8 are inversely related.

The amplifier shown in FIG. 2 comprises three stages of amplificationcomprising the transistors 30, 31 and 32 each connected in thecommon-emitter configuration. The input signal from a source 33 ofsignals to be amplified is applied to the base electrode 34 oftransistor 39 by means. of a resistor 35, while the D.-C. potential atthe base electrode of the transistor is established by a voltage dividercircuit comprising positive voltage source 36, negative voltage source37 and resistors 38 and 39 which connect sources 36 and 37,respectively, to base electrode 34. The input signal path is completedby means of the serial connection of resistor 40 and capacitor 41connected between the base electrode 34 and ground potential. Thecollector emitter bias on the transistor is established by means of aresistor 42 connected between the collector electrode 43 of transistor30 and source 36 and a resistor 44 connected between the emitterelectrode 45 of transistor 30 and negative voltage source 37. A junctiondiode 46 connected between the emitter electrode 45 of transistor 30 andground potential provides temperature stability.

The output signal from the first common emitter transistor amplifier 30is taken from the collector electrode '43 by means of a directconnection to the base electrode 48 of transistor amplifier 31. Theemitter electrode 49 of transistor 31 is established at approximatelythe same potential as the collector electrode 43 of transistor 30 bymeans of a Zener diode 50 connected between the emitter electrode 49 andground potential. The collector electrode 51 is biased by source 36through resistor 52.

In other applications, as explained above, p.13 H may be heldsubstantially constant by reducing the value The collector electrode 51of transistor 31 is coupled -to the base electrode 57 of the transistor32 of the third stage of amplification by means of two Zener diodes withthe cathode of the first Zener diode 55 connected to the collectorelectrode 51 of transistor 31 and the anode of the second Zener diode 56connected to the base electrode 57 of transistor 32. The anode of Zenerdiode 55 and the cathode of Zener diode 56 are connected together at acommon junction point 58 and the volt-age at this common junction pointis determined by means of the volt-age divider comprising voltage source36, resis to-r 52, negative voltage source 37, and resistor 59. A Zenerdiode 61 connected between the emitter electrode 60 and. groundpotential and a resistor 62 connected between negative voltage source37, and emitter electrode 60 establish the proper emitter electrodebias. The collector electrode 63 of transistor 32 is biased by means ofa connection from positive voltage source 36 through resistor 64.

At low values of output voltage the only feedback in this circuitry isprovided by resistor 69 connected between the collector electrode 63 oftransistor 32 and the base electrode 34 of transistor 30. Two additionalpairs of overall feedback paths between the collector electrode 63 oftransistor 32 and the base electrode 34 of transistor 30 are provided bytwo pairs of junction diodes 70-72 and 71-73. The anodes of diodes 70and 71 are connected to the collector electrode 63 of transistor 32 byresistors 75 and 76, respevtively, so that when predetermined positiveoutput voltage levels are reached the diodes will conduct as determinedby the threshold value established at the anode of each diode. Athreshold circuit comprising negative voltage source 37, potentiometer'80, and resistors 81 and 82' establishes the positive voltage back pathshunts the original feedback path provided by resistor 69 connectedbetween the collector electrode 63 of transistor 32 and the baseelectrode 34 of transistor 30 and provides additional feedback. When theoutput signal reaches the level of positive voltage at which diode 70conducts still another overall feedback circuit is connected in thecircuit, 5 is increased still more, and the gain is reduced again.

Similar circuitry is provided to insert two additional overall feedbackpaths when the input signal goes negative. These latter feedback pathscomprise diodes 72 and 73 which are connected to collector electrode 63of transistor 32 by means of resistors 86 and 87. Junction diodes 72 and73 are biased to conduct at the same magnitude of output level as diodes70 and 71, respectively, by means of threshold circuitry similar to thatused in connection with diodes 70 and 71.

To avoid the problem of instability created by these increases inoverall feedback as the output signal becomes greater in magnitude, inaccordance with this invention, the nominal gain p. of the amplifier isreduced at an intermediate value of output voltage magnitude so that thefeedback factor 1.[3 is kept within a range sufiicient to maintaincircuit stability and bandwidth. This is accomplished by means of alocal feedback path around transistor amplifier 32 so that when theoutput signal is greater in absolute magnitude than the level at whichdiodes 71 and 73 conduct either diode 90 or diode 91, depending whetherthe output signal is positive or' negative, will conduct thus connectingthe collector electrode 63 of transistor 32 to junction point 58 throughresistor 92.- This local feedback path around transistor 32 reduces thegainof the third amplifier stage thus reducing the nominal gain ,u. ofthe three stage amplifier. The reduction in the value of ,u compensatesfor the increasing values of overall feedback so that even when bothdiodes 70, 71 (or 72, 73) conduct the value of the feedback factor ;tfiis sufficiently reduced so that the amplifier is not rendered unstable.Thus in the embodiment of the invention shown in FIG. 2 the value ,LLdoes not precisely inversely follow the value of 5 feedback, but ratheris reduced so that the product ,uB, that is the feedback factor, is heldto a value at which the amplifier is stable. It should be recognized, ofcourse, that the embodiment of the invention shown in FIG. 2 is merelyan example of the application of the principles of the invention andthat ,u could be made to follow 8 in an inverse manner so that thefeedback factor would be maintained substantially constant.

To obtain a logarithmic input-output gain characteristic the followingcomponents and component values may be employed in the circuit of FIG.2. With these components and component values the direct current levelof the base electrode 34 of transistor is zero volt and the junction 58is also at zero direct current level. The diodes 71 and 73 conduct at +1volt and 1 volt, re-

spectively, and the diodes 70 and 72 conduct at +2 volts Transistors 30,31, 32 2Nl051 Diode 46 1N697 Diode 50 v. Zener 6 Diode 55 v. Zener 15Diode 56 v. Zener 8 Diode 61 v. Zener 9 Diode 70 1N696 Diode 71 1N6 96Diode 72 -1N696 Diode 73 1N696 Diode 90 1N696 Diode 91 1N696 Resistor30K Resistor 38 240K Resistor 39 120K Resistor 40 560 Resistor 42 430KResistor 44 10K Resistor 52 6.8K Resistor 59 8.2K Resistor 62 360Resistor 64 2.4K Resistor 69 100K Resistor 75 300K Resistor 76 2.7KResistor 81 12.7K Resistor 82 Meg 7.3 Resistor 86 300K Resistor 87 2.7KResistor 92 390 Capacitor 41 [Lf .005 Capacitor 93 pf 10 Capacitor 94 pf680 Voltage source 36 v +48 Voltage source 37 v 24 It is to beunderstood that the above-described arrangements are illustrative of theapplication of the principles of the invention. Numerous otherarrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. An amplifier for producing signal outputs nonlinearly related to thesignal input comprising an amplifier having a nominal gain ,u, anon-linear negative feedback network connected between the output andinput of said amplifier for feeding back to the input a fraction 3 ofthe output signal, said fraction ,6 varying with the signal level, andmeans for varying the nominal gain a in response to variations in signallevel in inverse proportion to the variations in 6.

2. An amplifier for producing signal outputs non-linear- 1y related tothe signal input comprising an amplifier having a nominal gain ,4, anon-linear negative feedback network connected between the output andinput of said amplifier for feeding back to the input a fraction 5 ofthe output signal, said fraction [3 increasing for predetermined higheroutput level signals, and means to reduce the nominal gain a in responseto said predetermined higher output level signals so that ,u and [3 areinversely related.

3. An amplifier for producing signal outputs non-linearly related to thesignal input comprising an amplifier having a nominal gain ,u, an inputand an output, and'at least two stages of amplification, one of thestages of amplification having a local non-linear negative feedbacknetwork connected between the output and input of the stage for feedingback to the input of the stage a fraction ,6, of the output signal fromthe stage, a non-linear negative feedback network connected between theoutput and input of said amplifier for feeding back to the input afraction ,8 of the output signal, said fraction [3 increasing inresponse to predetermined higher levels of the output signal to reducethe overall gain of said amplifier circuit, and means to increase thefeedback fraction ,8 of said local feedback path in response to saidpredetermined higher levels of the output signal to reduce the gain ofthe stage of amplification having local feedback to reduce the nominalgain ,u of said amplifier so that the feedback factor 43 is maintainedsubstantially constant.

4. An amplifier circuit having a non-linear input-output characteristiccomprising an amplifier having an. input terminal, an output terminal,and at least two stages of amplification of nominal gain a, one of saidstages having a non-linear local feedback network connected between itsinput and output for feeding back a fraction [3 of the output signalfrom the output of said stage to its input, said feedback fraction Bbeing zero below a predetermined magnitude of the output signal fromsaid stage of amplification, a non-linear overall feedback networkconnected between said input and output terminals of said amplifier forfeeding back to the input a fraction B of the output signal whichfraction increases in discrete steps when the output signal exceedspredetermined levels, and means to increase the feedback fraction ,8 ofsaid local feedback path at a level'of the output signal intermediatesaid predetermined levels at which the feedback fraction [3 of theoverall feedback path is increased.

5. An amplifier circuit having a non-linear input-output characteristiccomprising an amplifier having an input terminal, an output terminal,and at least two stages of amplification having a nominal gain ,u, anoverall feedback network connected between said input and outputterminals of said amplifier comprising a resistor connected to feed backa predetermined fraction of the output signal, a second overall feedbacknetwork connected between said input and output terminals comprising afirst pair of biased switching diodes each of which conducts ata firstpredetermined level of the output signal to increase the feedbackfraction of the output signal fed back to the input of the amplifier afirst of said first pair of diodes conducting when the output signal isof positive polarity and a second conducting when the output signal isof negative polarity, a third overall feedback network connected betweensaid input and output terminals comprising a second pair of biasedswitching diodes each of which conducts at a second predetermined levelof the output signal higher than said first predetermined level toincrease the feedback fraction of the output signal fed back to theinput of the amplifier, a first of said second pair of diodes conductingwhen the output signal is of positive polarity and a second conductingwhen the output signal is of negative polarity, and a local feedbacknetwork comprising a pair of biased switching diodes each of whichconducts at a third predetermined level of the output signalintermediate said first and second predetermined levels to feed back afraction of the output signal to the input of the last stage ofamplification, a

References Cited by the Examiner UNITED STATES PATENTS 2,210,503 8/1940Shaw 330-99 X 2,811,591 10/1957 Kennedy 330-103 X 8 2,909, 20 10/1959Graef 330-103X 3,092,729 6/1963 Gray 33028X 3,124,759 3/1964 Dahlberg330-28X FOREIGN PATENTS 261,000 9/ 1949 Switzerland.

ARTHUR GAUSS, Primary Examiner.

M. LEE, Assistant Examiner.

1. AN AMPLIFIER FOR PRODUCING SIGNAL OUTPUTS NONLINEARLY RELATED TO THESIGNAL INPUT COMPRISING AN AMPLIFIER HAVING A NOMINAL GAIN U, ANON-LINEAR NEGATIVE FEEDBACK NETWORK CONNECTED BETWEEN THE OUTPUT ANDINPUT OF SAID AMPLIFIER FOR FEEDING BACK TO THE INPUT A TRACTION B OFTHE OUTPUT SIGNAL, SAID FRACTION B VARYING WITH THE SIGNAL LEVEL, ANDMEANS FOR VARYING THE NOMINAL GAIN U IN RESPONSE TO VARIATIONS IN SIGNALLEVEL IN INVERSE PROPORTION TO THE VARIATIONS IN B.